At $1.66 per serving, this is an inexpensive, not to mention simplified, version of the traditional minced lamb classic. It's also not too different from the Shepherd's Pie my mother used to put together on a cold night; hers had cheese on top but the creamy mashed potatoes make this filling dish rich enough.
Preheat the oven to 350 degrees. In an oven-proof skillet over medium-high heat, sauté the onion and garlic in the oil. Put aside. Brown the ground beef in the skillet over medium-high heat, seasoning with the salt and rosemary, breaking up large chunks of meat. Pour off excess drippings and add the onions and garlic back in. On low heat, add in the Worcestershire and peas, and allow to cook 5 more minutes. (Special note: If you happen to have some beer or wine nearby, now's a good time to pour a capful in; if not, don't worry it will still be tasty!) Remove from the heat, cover, and set aside.
Meanwhile, boil the potatoes (skin on) in salted water for 25 minutes. Drain water, add in the butter and sour cream, and go to town with the potato masher. Season with salt and pepper to taste. When the mashed potatoes are smooth enough for your liking, spread them on top of the ground beef in the skillet and use a fork to make some pretty crosshatching marks.
Bake for 20 minutes, until the mashed potatoes begin to get golden in spots. Allow to rest for a few minutes. Slice into servings and serve hot.